Question

I have two tables users and *activation_details*

Users table has these data. uid is primary key.

uid     |    user_name
______________________
 1     |    John Smith
 2     |    Mary Smith
 3     |    Nancy Smith
 4     |    Agent Smith

activation_details has these data

aid     |    is_activated   |  user_id
______________________________________
1      |         0         |     0
2      |         1         |     4
3      |         1         |     1
4      |         1         |     777 

Please note that user id 777 is not in the users table.

I need the result in this format

aid     |    is_activated   |  user_name      
______________________________________________
1       |         0         |     0
2       |         1         |     Agent Smith
3       |         1         |     John Smith
4       |         1         |     777 

If user id is present in the users table then username has to be displayed, else the user_id itself has to be displayed.

I tried some thing like this

SELECT aid, is_activated, user_id, users.user_name from activation_details, users WHERE users.uid = user_id

It gives this output, which is not useful.

aid     |    is_activated   |  user_name      | user_id
________________________________________________________ 
2       |         1         |     Agent Smith |     4 
3       |         1         |     John Smith  |     1

Is there anyway to get this output using mysql only, I can do it with multiple queries in PHP, but that is not what I am looking.

aid     |    is_activated   |  user_name
______________________________________
1       |         0         |     0
2       |         1         |     Agent Smith
3       |         1         |     John Smith
4       |         1         |     777 

Answer 1

You want to use an outer join in combination with the coalesce() function (which returns the first non-null value in the selected columns) like this:

SELECT 
    a.aid, 
    a.is_activated, 
    a.user_id, 
    coalesce(b.user_name, a.user_id, 0) as User
from 
    activation_details a
        left outer join users b
            on a.user_id=b.uid

Edit: I added one more check for the coalesce() function which will return a 0 for the User in case both the user_name and user_id fields are null values.

Testing done in mysql:

mysql> select * from first
    -> ;
+------+-------+
| id   | title |
+------+-------+
|    1 | aaaa  |
|    2 | bbbb  |
|    3 | cccc  |
+------+-------+
3 rows in set (0.00 sec)

mysql> select coalesce('a', id) from first
    -> ;
+-------------------+
| coalesce('a', id) |
+-------------------+
| a                 |
| a                 |
| a                 |
+-------------------+
3 rows in set (0.00 sec)

mysql> select coalesce('a', 1) from first;
+------------------+
| coalesce('a', 1) |
+------------------+
| a                |
| a                |
| a                |
+------------------+
3 rows in set (0.00 sec)

mysql> select coalesce(null, 1) from first;
+-------------------+
| coalesce(null, 1) |
+-------------------+
|                 1 |
|                 1 |
|                 1 |
+-------------------+
3 rows in set (0.00 sec)